Abstract:Dopamine (DA) transmission mediates numerous aspects of behavior. Although DA release is strongly linked to firing of DA neurons, recent developments indicate the importance of presynaptic modulation at striatal dopaminergic terminals. The endocannabinoid (eCB) system regulates DA release and is a canonical gatekeeper of goal-directed behavior. Here we report that extracellular DA increases induced by selective optogenetic activation of cholinergic neurons in the nucleus accumbens (NAc) are inhibited by CB1 agonists and eCBs. This modulation requires CB1 receptors on cortical glutamatergic afferents. Dopamine increases driven by optogenetic activation of prefrontal cortex (PFC) terminals in the NAc are similarly modulated by activation of these CB1 receptors. We further demonstrate that this same population of CB1 receptors modulates optical self-stimulation sustained by activation of PFC afferents in the NAc. These results establish local eCB actions on PFC terminals within the NAc that inhibit mesolimbic DA release and constrain reward-driven behavior.

Abstract:The vast majority of neurons within the striatum are GABAergic medium spiny neurons (MSNs), which receive glutamatergic input from the cortex and thalamus, and form two major efferent pathways: the direct pathway, expressing dopamine D1 receptor (D1R-MSNs), and the indirect pathway, expressing dopamine D2 receptor (D2R-MSNs). While molecular mechanisms of MSN degeneration have been identified in animal models of striatal damage, the molecular factors that dictate a selective vulnerability of D1R-MSNs or D2R-MSNs remain unknown. Here, we combined genetic, chemogenetic, and pharmacological strategies with behavioral and neurochemical analyses, and show that the pool of cannabinoid CB1 receptor (CB1R) located on corticostriatal terminals efficiently safeguards D1R-MSNs, but not D2R-MSNs, from different insults. This cell-specific response relies on the regulation of glutamatergic signaling, and is independent from the CB1R-dependent control of astroglial activity in the striatum. These findings define cortical CB1R as a pivotal synaptic player in dictating a differential vulnerability of D1R-MSNs versus D2R-MSNs, and increase our understanding of the role of coordinated cannabinergic-glutamatergic signaling in establishing corticostriatal circuits and its dysregulation in neurodegenerative diseases.

Abstract:Cellular activity in the brain depends on the high energetic support provided by mitochondria, the cell organelles which use energy sources to generate ATP. Acute cannabinoid intoxication induces amnesia in humans and animals, and the activation of type-1 cannabinoid receptors present at brain mitochondria membranes (mtCB1) can directly alter mitochondrial energetic activity. Although the pathological impact of chronic mitochondrial dysfunctions in the brain is well established, the involvement of acute modulation of mitochondrial activity in high brain functions, including learning and memory, is unknown. Here, we show that acute cannabinoid-induced memory impairment in mice requires activation of hippocampal mtCB1 receptors. Genetic exclusion of CB1 receptors from hippocampal mitochondria prevents cannabinoid-induced reduction of mitochondrial mobility, synaptic transmission and memory formation. mtCB1 receptors signal through intra-mitochondrial Galphai protein activation and consequent inhibition of soluble-adenylyl cyclase (sAC). The resulting inhibition of protein kinase A (PKA)-dependent phosphorylation of specific subunits of the mitochondrial electron transport system eventually leads to decreased cellular respiration. Hippocampal inhibition of sAC activity or manipulation of intra-mitochondrial PKA signalling or phosphorylation of the Complex I subunit NDUFS2 inhibit bioenergetic and amnesic effects of cannabinoids. Thus, the G protein-coupled mtCB1 receptors regulate memory processes via modulation of mitochondrial energy metabolism. By directly linking mitochondrial activity to memory formation, these data reveal that bioenergetic processes are primary acute regulators of cognitive functions.

Abstract:The dorsal striatum is a major input structure of the basal ganglia and plays a key role in the control of vital processes such as motor behavior, cognition, and motivation. The functionality of striatal neurons is tightly controlled by various metabotropic receptors. Whereas the Gs/Gi-protein-dependent tuning of striatal neurons is fairly well known, the precise impact and underlying mechanism of Gq-protein-dependent signals remain poorly understood. Here, using different experimental approaches, especially designer receptor exclusively activated by designer drug (DREADD) chemogenetic technology, we found that sustained activation of Gq-protein signaling impairs the functionality of striatal neurons and we unveil the precise molecular mechanism underlying this process: a phospholipase C/Ca2+/proline-rich tyrosine kinase 2/cJun N-terminal kinase pathway. Moreover, engagement of this intracellular signaling route was functionally active in the mouse dorsal striatum in vivo, as proven by the disruption of neuronal integrity and behavioral tasks. To analyze this effect anatomically, we manipulated Gq-protein-dependent signaling selectively in neurons belonging to the direct or indirect striatal pathway. Acute Gq-protein activation in direct-pathway or indirect-pathway neurons produced an enhancement or a decrease, respectively, of activity-dependent parameters. In contrast, sustained Gq-protein activation impaired the functionality of direct-pathway and indirect-pathway neurons and disrupted the behavioral performance and electroencephalography-related activity tasks controlled by either anatomical framework. Collectively, these findings define the molecular mechanism and functional relevance of Gq-protein-driven signals in striatal circuits under normal and overactivated states. SIGNIFICANCE STATEMENT: The dorsal striatum is a major input structure of the basal ganglia and plays a key role in the control of vital processes such as motor behavior, cognition, and motivation. Whereas the Gs/Gi-protein-dependent tuning of striatal neurons is fairly well known, the precise impact and underlying mechanism of Gq-protein-dependent signals remain unclear. Here, we show that striatal circuits can be 'turned on' by acute Gq-protein signaling or 'turned off' by sustained Gq-protein signaling. Specifically, sustained Gq-protein signaling inactivates striatal neurons by an intracellular pathway that relies on cJun N-terminal kinase. Overall, this study sheds new light onto the molecular mechanism and functional relevance of Gq-protein-driven signals in striatal circuits under normal and overactivated states.

Abstract:Stressful events can generate emotional memories linked to the traumatic incident, but they also can impair the formation of nonemotional memories. Although the impact of stress on emotional memories is well studied, much less is known about the influence of the emotional state on the formation of nonemotional memories. We used the novel object-recognition task as a model of nonemotional memory in mice to investigate the underlying mechanism of the deleterious effect of stress on memory consolidation. Systemic, hippocampal, and peripheral blockade of cannabinoid type-1 (CB1) receptors abolished the stress-induced memory impairment. Genetic deletion and rescue of CB1 receptors in specific cell types revealed that the CB1 receptor population specifically in dopamine beta-hydroxylase (DBH)-expressing cells is both necessary and sufficient for stress-induced impairment of memory consolidation, but CB1 receptors present in other neuronal populations are not involved. Strikingly, pharmacological manipulations in mice expressing CB1 receptors exclusively in DBH+ cells revealed that both hippocampal and peripheral receptors mediate the impact of stress on memory consolidation. Thus, CB1 receptors on adrenergic and noradrenergic cells provide previously unrecognized cross-talk between central and peripheral mechanisms in the stress-dependent regulation of nonemotional memory consolidation, suggesting new potential avenues for the treatment of cognitive aspects on stress-related disorders.

Abstract:The endocannabinoid system (ECS) is abundantly expressed in the brain. This system regulates a plethora of physiological functions and is composed of cannabinoid receptors, their endogenous ligands (endocannabinoids), and the enzymes involved in the metabolism of endocannabinoids. In this review, we highlight the new advances in cannabinoid signaling, focusing on a key component of the ECS, the type-1 cannabinoid receptor (CB 1). In recent years, the development of new imaging and molecular tools has demonstrated that this receptor can be distributed in many cell types (e.g., neuronal or glial cells) and intracellular compartments (e.g., mitochondria). Interestingly, cellular and molecular effects are differentially mediated by CB 1 receptors according to their specific localization (e.g., glutamatergic or GABAergic neurons). Moreover, this receptor is expressed in the periphery, where it can modulate periphery-brain connections. Finally, the better understanding of the CB 1 receptor structure led researchers to propose interesting and new allosteric modulators. Thus, the advances and the new directions of the CB 1 receptor field will provide new insights and better approaches to profit from its interesting therapeutic profile.

Abstract:The endocannabinoid system is the target of the main psychoactive component of the plant Cannabis sativa, the Delta9 -tetrahydrocannabinol (THC). This system is composed by the cannabinoid receptors, the endogenous ligands, and the enzymes involved in their metabolic processes, which works both centrally and peripherally to regulate a plethora of physiological functions. This review aims at explaining how the site-specific actions of the endocannabinoid system impact on memory and feeding behavior through the cannabinoid receptors 1 (CB1 R). Centrally, CB1 R is widely distributed in many brain regions, different cell types (e.g. neuronal or glial cells) and intracellular compartments (e.g. mitochondria). Interestingly, cellular and molecular effects are differentially mediated by CB1 R according to their cell-type localization (e.g. glutamatergic or GABAergic neurons). Thus, understanding the cellular and subcellular function of CB1 R will provide new insights and aid the design of new compounds in cannabinoid-based medicine. Also watch the Video Abstract.

Abstract:The CB1 cannabinoid receptor, the main molecular target of endocannabinoids and cannabis active components, is the most abundant G protein-coupled receptor in the mammalian brain. In particular, the CB1 receptor is highly expressed in the basal ganglia, mostly on terminals of medium-sized spiny neurons, where it plays a key neuromodulatory function. The CB1 receptor also confers neuroprotection in various experimental models of striatal damage. However, the assessment of the physiological relevance and therapeutic potential of the CB1 receptor in basal ganglia-related diseases is hampered, at least in part, by the lack of knowledge of the precise mechanism of CB1 receptor neuroprotective activity. Here, by using an array of pharmacological, genetic and pharmacogenetic (designer receptor exclusively activated by designer drug) approaches, we show that (1) CB1 receptor engagement protects striatal cells from excitotoxic death via the phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin complex 1 pathway, which, in turn, (2) induces brain-derived neurotrophic factor (BDNF) expression through the selective activation of BDNF gene promoter IV, an effect that is mediated by multiple transcription factors. To assess the possible functional impact of the CB1/BDNF axis in a neurodegenerative-disease context in vivo, we conducted experiments in the R6/2 mouse, a well-established model of Huntington's disease, in which the CB1 receptor and BDNF are known to be severely downregulated in the dorsolateral striatum. Adeno-associated viral vector-enforced re-expression of the CB1 receptor in the dorsolateral striatum of R6/2 mice allowed the re-expression of BDNF and the concerted rescue of the neuropathological deficits in these animals. Collectively, these findings unravel a molecular link between CB1 receptor activation and BDNF expression, and support the relevance of the CB1/BDNF axis in promoting striatal neuron survival.Cell Death and Differentiation advance online publication, 20 February 2015; doi:10.1038/cdd.2015.11.